What is the test for reducing murder to voluntary manslaughter?

If the defendant commits an unlawful, intentional killing but lacks malice, he or she is guilty of the lesser-included offense of voluntary manslaughter. ( 192; People v. Barton (1995) 12 Cal.4th 186, 199.) Two circumstances may negate malice and thus reduce murder to voluntary manslaughter: (1) when the defendant acts in a "sudden quarrel or heat of passion" ( 192, subd. (a)), or (2) when the defendant kills with an unreasonable but good faith belief in having to act in self-defense - an "imperfect self-defense." (People v. McCoy (2001) 25 Cal.4th 1111, 1116; People v. Barton, supra, at p. 199.)